The invention relates to the intimate contacting of a gas and a loose granular solid material for purpose of chemically or physically treating one or both of the gaseous and solid substances, for example: to filter fine particulate matter (xe2x80x9cdustxe2x80x9d)from the gas; to effect a chemical change in gas or solid; to remove a chemical constituent of the gas by absorption or adsorption; or to heat a cold gas by contact with a hot solid. The invention also relates to the countercurrent contacting of a gas and a granular solid material.
An old idea is to treat a gas and a granular solid material by causing the gas to flow in the horizontal direction across a bed of the solid material disposed in a xe2x80x9cpanelxe2x80x9d that has often been tall in comparison with its width in the direction of gas flow. Often, the xe2x80x9cpanel bedxe2x80x9d has been held in place by louvered walls that resembled venetian blinds. My U.S. Pat. No. 4,006,533 (Feb. 8, 1977) cites early art and is incorporated by reference in the instant application.
Some designs called for continuous or intermittent motion of granular material downward through the panel, fresh material being supplied at the panel""s top and xe2x80x9cspentxe2x80x9d material being withdrawn at its bottom. A representative recent proposal along this line is to be found in U.S. Pat. No. 5,527,514 (Jun. 18, 1996). U.S. Pat. No. 4,017,278 (Apr. 12, 1977) provided performance data for a gas-filtration device of this kind. The panel contained xe2x80x9cgravelxe2x80x9d 2 to 12 mm in size. Superficial velocity of dusty gas approaching the panel was 25 to 100 cm/s. Herein, superficial velocity=rate of gas flow divided by the projected vertical frontal area of the panel (panel heightxc3x97panel width). Downward speed of the gravel mass was 30 cm/hr. Dust content of filtered gas ranged from xcx9c25 to xcx9c150 milligrams per normal cubic meter (mg/Nm3). Dust that was captured by filtration accumulated within the gravel bed, not upon the gas-entry surfaces that it presented. In other words, the device filtered the dust by what practitioners term a xe2x80x9cdeep-bed filtration mechanism.xe2x80x9d
In other designs, granular material was stationary much of the time. These designs filtered dust from gas by accumulating a cake of the dust (a xe2x80x9cfilter cakexe2x80x9d)upon gas-entry faces of granular material retained in a panel bed. Such designs are capable of providing filtered gas containing levels of residual dust comparable to that provided by fabric filtration. Both a panel bed of this type and a fabric filter employ a xe2x80x9csurface filtration mechanism,xe2x80x9d in which the filter cake is in fact the filtration medium; the primary function of either granular bed or fabric is to support the cake. Means are provided for intermittent renewal of gas-entry faces through removal of a moiety of the granular material from these faces together with accumulated dust. Means for outwardly tipping louvers that support gas-entry faces and for drawing plows horizontally along the faces have been proposed. U.S. Pat. No. 3,800,508 (Apr. 2, 1974) not only provided means for pivoting louvers but also employed a gas-entry velocity at gas-entry faces sufficient to support the faces at an angle steeper than the dynamic angle of repose of the granular material, momentary interruption of gas flow produced a spill of this material along with filter-cake.
My U.S. Pat. No. 3,296,775 (Jan. 10, 1967) disclosed a puff-back method for renewing gas-entry faces of a panel bed of the type wherein louvers support gas-entry portions of the bed. Puff-back entailed creation of a reverse transient surge flow to produce en masse displacement of granular material toward gas-entry faces of this material. My U.S. Pat. No. 4,006,533 (Feb. 8, 1977) specified a reverse transient surge flow of a more particular character, whose discovery made possible development of a practical panel-bed filter employing a surface filtration mechanism for cleaning a dusty gas (K. C. Lee, I. Rodon, M. S. Wu, R. Pfeffer, and A. M. Squires, The Panel Bed Filter, EPRI AF-560, Electric Power Research Institute, Palo Alto Calif., May 1977, I. Rodon, K. C. Lee, R. Pfeffer, and A. M. Squires, Panel Bed Filtration Data for Three Dusts at 150xc2x0 C., paper 79-56.5 presented at meeting of Air Pollution Control Association, Cincinnati, Ohio, June 1979, A. M. Squires, K. C. Lee, and R. Pfeffer, The Panel Bed: A Fluid-Solid Contacting Device Exploiting a New Mode of Soil Failure, paper presented at POWTECH 81, Birmingham, England, March 1981). Operation of a panel-bed filter is cyclic, an interval of filtration alternating with a puff-back that removes both filter cakes and a moiety of sand lying directly beneath the cake, thereby renewing the bed""s gas-entry faces. During an interval of filtration, paralleling the formation of the filter cake is an increase in pressure drop in the gas flowing across the bed. This pressure drop cannot be allowed to increase without limit, for two reasons: an unduly large pressure drop would increase cost for gas-compression beyond an economic limit and would impose a force upon the filter cake sufficient to break off chunks of the cake, driving these deep into the granular bed and harming filtering efficiency. Although the object of face-renewal is to present a new free face, it is inadvisable to employ such a strong puff-back as to yield an absolutely clean face. In operation of panel-bed filters, experience has taught that a clean face does not filter as well as a somewhat dirty face. In a subsequent filtering interval, I believe, a new filter cake forms quicker upon a dirty face than upon a strictly clean one.
Commercial-scale panel-bed-filter modules have now successfully cleaned hot gaseous products of combustion of both coal and wood waste, hot gas from cement production, and hot gas from electrometallurgical manufacture of ferrosilicon (this latter gas, containing a fine silica fume, is particularly difficult to clean). Typically, dust in the cleaned gas amounted to less than 5 mg/Nm3.
In the tests, ordinary sand served as the granular filtration material. Suitably, the sand was about 0.15-0.45 mm in size. In small-scale tests at elevated temperature (e.g., 150 to 500xc2x0 C.), I found this size, for dusts studied so far, to be substantially the largest sand size upon which a filter cake of good integrity can accumulate. Use of sands of smaller sizes affords filtered gas at lower residual dust remaining, at cost of either lower throughput or higher pressure drop in the filtered gas. For a given size of sand, to allow a filter cake to form, there are limits upon the velocity of gas entering a free face of the sand. For the 0.15-0.45 mm sand, in tests filtering a number of dusts at about 150-200xc2x0 C., preferred gas-entry velocities have ranged from xcx9c16 to xcx9c28 cm/s (superficial velocities, from xcx9c8 to xcx9c14 cm/s). Herein, gas-entry velocity=rate of gas flow divided by the nominal total area of the free sand faces upon which a filter cake can accumulate. Since the profile of a gas-entry face, seen in vertical cross-section, is poorly defined, it is convenient to define a nominal area, thus:
Gas-entry face area=(straight-line distance between the face""s outer and inner edges in the direction perpendicular to the edges)xc3x97(the mean of the horizontal lengths of these edges).
(Notice that this definition applies, regardless whether the face""s edges are straight or curved in the horizontal direction. In state-of-the-art designs, edges are straight, but louvers that are circular in plan may be useful in some applications.) In general, a lower gas-entry velocity is preferable the smaller the size of dust to be filtered or the less cohesive the dust.
Parenthetically, I note that both larger granular material sizes and higher gas-entry velocities may be specified for panel-bed applications wherein a clean gas is treated by a contact with the granular material.
In a test of a commercial-scale panel-bed filter module, wood ash was filtered from gas at 200xc2x0 C. emitted by a wood-waste boiler (H. Risnes and O. K. Sxc3x8nju, Evaluation of a novel granular bed filtration system for high temperature applications, paper presented at conference on Progress in Thermal and Biomass Conversion, Tyrol, Austria, 2000). The module comprised two panel beds, each 600 mm in width and 3000 mm in height. For each bed, the projected vertical frontal area was about twice the nominal area of gas-entry faces. Thus, superficial gas velocity was about one-half the gas-entry velocity. Gas-entry sand faces received and filtered dusty gas supplied to a space surrounding the module. In cooperation with nonporous side panels, the two panel beds enclosed a central space in which cleaned gas moved upward toward an outlet. On average, residual dust in filtered gas was 1.7 mg/Nm3. This excellent filtration performance has been confirmed in tests of an installation comprising 27 substantially identical modules receiving gaseous products of combustion at 190xc2x0 C. from a 5 Mw wood-waste boiler (the tests having been marred, however, by a poor choice of apparatus for separating puff-backed sand from the filtered wood ash).
At its present stage of development, the panel-bed filter with puff-back is an economically attractive choice for many applications (especially, for example, for removing ash from wood-waste combustion or silica fume arising from FeSi production). An important parameter for judging a device for removing dust from a gas is the area of ground that it occupies. The xe2x80x9cfootprintxe2x80x9d of a state-of-the-art panel-bed filter is approximately 0.13 square meter per 1,000 cubic meters per hour of dusty gas to be filtered. In contrast, the footprint of a high-efficiency electrostatic precipitator is about 0.37 m2/(Km3/hr) of gas flow. A fabric filter""s footprint is of course even larger.
There is, however, room for improving the panel-bed filter. The fabricator of modules for tests on a wood-waste boiler, described above, complained at the large number of individual louver elements needing to be assembled. A higher gas-treating capacity per louver would constitute an advance in the art. In addition, cost of filter modules is a major part of the total cost of a filter installation. Higher superficial gas velocity (hence, larger gas-treating capacity per filter module and reduction in number of modules needed for a given application) would also advance the art.
A broad objective of the instant invention is to reduce the cost of a panel-bed installation by reducing numbers of both louvers and panel-bed modules required to meet a given gas-treating capacity. The instant invention can achieve a four-fold or greater increase in the capacity of a panel bed of given projected frontal area. The invention can achieve a four-fold or greater decrease in the number of individual louvers that must be manufactured and assembled.
An increase in panel-bed-filter capacity requires a concomitant increase in spaces required for supplying dusty gas and for carrying away cleaned gas. Accordingly, the footprint of a panel-bed installation does not decrease in the same proportion as the increase in module capacity. The instant invention can shrink the footprint from the aforementioned 0.13 m2/(Km3/hr) to approximately 0.09 m2/(Km3/hr).
Undertaking a series of tests of louvers of novel design, I have been surprised to discover that puff-back is more effective in clearing sand from louvers that incline inwardly upwardly from nearby their outer edges. A panel bed employing such louvers, in addition, presents larger gas-entry faces relative to its projected frontal area. The bed, accordingly, permits use of a higher superficial velocity for a given gas-entry velocity; i.e., the capacity of the bed for treating gas or granular material is higher than the capacity of a panel bed employing state-of-the-art louvers at the given gas-entry velocity.
A primary objective of puff-back is to cause en masse displacement of granular material toward a louver""s outer edge. A uniform displacement creates a spill of the material that arises uniformly across its gas-entry face. In a state-of-the-art panel-bed filter, the displacement is far from uniform (although its success in filtering dusty gases shows that the displacement is sufficiently uniform to achieve cleaned gases containing less than 5 mg/Nm3). I have been surprised to discover that en masse displacement produced by puff-back is more uniform for louvers that incline upwardly inwardly from nearby their outer edges than it is for louvers employed hitherto.
Louvers sloping upwardly from nearby their outer edges have another significant advantage. They lend themselves to a renewal by a sharp vertically upward motion of the louvers, such as may be effected by a sharp upward blow of a hammer upon a panel-bed assembly. The slope of louvers of the instant invention cooperates with a sharp upward movement to produce a highly uniform en masse displacement of granular material toward panel-bed gas-entry faces. In principle, in comparison with puff-back, a hammer up-blow upon a panel bed fitted with upwardly inwardly inclined louvers can effect greater uniformity of displacement of granular material respecting a large number of supporting louvers.
Such uniformity of the displacement affords more uniform spills of sand from the plurality of gas-entry faces of the panel bed than puff-back can achieve. Even taking extreme care in design and manufacture, I have been unable with puff-back to achieve exact uniformity of spill of xe2x80x9cspentxe2x80x9d granular material from each gas-entry face of a panel bed. For louvers of substantially identical geometry, substantially exact uniformity of spill is afforded by sharp-upward-motion face renewal. Such uniformity is especially important for the good performance of a panel bed whose purpose is to effect countercurrent contact of a gas and a granular material.
For convenience of reference, I sometimes use the term xe2x80x9chammer-blow renewal of gas-entry facesxe2x80x9d; but it should be understood that means other than a hammer blow may induce a suitable, momentary, sharp vertical movement of supporting louvers.
An object of the invention is to provide an improved method and apparatus for the chemical and physical treatment of at least one of a gas and a granular solid material brought into contact.
Another object is to provide an improved method and apparatus for bringing a gas and a granular material into intimate contact.
Another object is to achieve countercurrenticity in the contacting of a gas and a granular material.
Another object is to provide an improved filter for removing fine particulate matter (xe2x80x9cdustxe2x80x9d) from a gas.
Another object is to provide an improved technique for periodically removing granular material adjacent to gas-entry faces of a panel-bed gas-powder contactor.
Another object is to provide a filter or solid contactor for gas at an extremely elevated temperature.
My invention relates to an improved method of contacting gas and granular material with each other to effect physical or chemical treatment of at least one of them. Granular material is arranged in a panel bed having a plurality of transversely disposed, upwardly spaced, gas-entry portions separated by interposed supporting louvers having outer and inner edges. Gas-entry portions have gas-entry faces that are substantially contiguous with these outer edges. The louvers slope inwardly and upwardly from nearby their outer edges. The panel bed has gas-exit portions spaced horizontally apart from the inner edges. The bed communicates vertically with a superjacent space containing a supply of the granular material. Gas is caused to flow forwardly in a substantially continuing flow during the aforementioned treatment through the gas-entry portions of the granular material bed and outwardly from the gas-exit portions to effect treatment of gas or granular material or both. Thereafter, gas-entry faces are renewed by causing en masse displacement of granular material respecting the supporting louvers, thereby causing material to fall away from each gas-entry face. The en masse displacement is effected either by action of puff-back or by action of hammer-blow renewal. The action causes granular material to descend by action of gravity from the supply space to make good losses of granular material from the gas-entry faces.
My invention also relates to an improved apparatus for contacting gas and granular material with each other to effect physical or chemical treatment of at least one of them. There is provided a pair of upwardly extending, horizontally spaced-apart perforate retaining walls, with gravity means, automatically responding to demand, for supplying a loose granular material into the space between the walls. There is a plurality of granular-material support louvers each adjacent a perforation of a first perforate wall, each louver being arranged to extend outwardly from below its adjacent perforation and into a gas-inlet compartment in communication with perforations of the first wall. Each louver slopes inwardly and upwardly from nearby its outer edge. The louvers cooperate to support and expose to the gas-inlet compartment a plurality of free faces of the granular material and to retain the material in the aforementioned space. A gas-outlet compartment is in communication with perforations of the second perforate wall. There is an inlet for admitting gas into the inlet compartment, and an outlet for removing gas from the outlet compartment. Means are provided for periodically effecting en masse displacement of the granular material respecting the supporting louvers, causing a moiety of granular material to fall away from each free face and creating a demand for supply of the granular material from the aforementioned gravity means, to make good losses of material from the free faces. Two alternative means are contemplated for effecting the en masse displacement. A first means furnishes a reverse transient surge flow of gas across the panel bed (a puff-back). A second means causes the support louvers to move vertically in a sharp upward motion.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.